1. Field of the Invention
The present invention relates to a method for preparing grease having excellent low noise properties. More specifically, the present invention pertains to a method for preparing grease which is suitable for sealing in motorcar parts for electric motor-operated devices such as an alternator, a solenoid-operated clutch for car air conditioners, an idle pulley, an intermediate pulley, an electric fan motor, a fluid coupling, a water pump, a distributor and a starter one-way clutch; rolling bearings, for instance, rolling bearings used at a high temperature and a high rotational speed such as auxiliary machinery for engines, rolling bearings used in household electric articles at a high temperature and a high rotational speed such as cleaner motors and air conditioner fan motors, rolling bearings used in spindle motors for recording devices such as computer hard disks and CD-R's and those for other widely used motors.
2. Description of the Prior Art
Principal factors for the improvement of the low noise properties of grease are, for instance, to uniformly disperse a thickener in the grease and to eliminate impurities present therein. Thickeners for greases may be classified into those completely soluble in a base oil and those hardly soluble therein when the temperature is raised during the preparation of the grease. Urea thickeners which are reaction products of an isocyanate and an amine are also classified into those completely soluble in a base oil and those hardly soluble therein.
As an example, Japanese Un-Examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 63-162790 discloses a means for the improvement of the low noise properties of completely soluble urea thickener-containing grease. More specifically, this patent discloses a method which comprises the steps of dissolving urea grease comprising a specific combination of ingredients in a lubricating oil and then passing the resulting solution through a filter to thus remove the impurities included therein. Since the thickener is completely dissolved in a base oil, this method may ensure the uniform dispersion of the thickener and also permits the removal of the impurities present therein by passing it through a filter. On the other hand, improvement of the low noise properties of hardly soluble urea thickener-containing grease is disclosed in Japanese Patent Application Serial No. Sho 63-153654 (J.P. KOKAI Hei 2-4895), which discloses that a thickener component can uniformly be dispersed in a base oil and the low noise properties of the resulting grease can thus be improved by subjecting the grease to a milling treatment after the completion of the reaction process.
In addition, J.P. KOKAI No. Hei 3-190996 discloses a method in which the low noise properties of grease can considerably be improved by using a mixing head as a reaction vessel and by reacting the isocyanate with the amine under a high pressure condition to thus uniformly disperse the resulting urea thickener in the grease.
However, recent grease has been required to satisfy increasingly severe requirements for low noise properties and the conventional methods have not been able to prepare any grease having the required low noise properties. For instance, in the method disclosed in Japanese Patent Application Serial No. Sho 63-153654 (J.P. KOKAI No. Hei 2-4895), the size of the thickener particles in the urea grease is determined simultaneous with the micelle formation during the reaction and therefore, it is impossible to further divide the resulting huge micelles into fine particles to thus obtain a uniformly dispersion by a subsequent milling treatment. For this reason, the conventional methods have never been able to provide any grease satisfying these severe requirements for low noise propertes.
Moreover, the method disclosed in J.P. KOKAI No. Hei 3-190996 permits the production of grease, which satisfies the foregoing severe requirements for low noise properties. However, the method requires a tremendous labor and is also very expensive.
In addition, the method disclosed in J.P. KOKAI No. Sho 63-162790 cannot be applied to the insoluble urea thickener-containing grease.